Receptor mediated inotropic responses in the 5-day-old embryonic chick heart have been shown to be markedly impaired by pretreatment with an impure preparation of phospholipase C. This preparation will be purified to remove contaminants and examined for its capacity to affect the responses to the same agonists (isoproterenol, acetylcholine, angiotensin II, 4-methylhistamine) employed in studies with impure enzyme. Again specificity of effect will be assessed by means of the receptor protection technic. Membrane fractions of hearts will be prepared and examined for binding of the ligands (125I) iodohydroxybenzylpindolol and/or (3H) dihydroalprenolol before and after treatment with the purified enzyme. These studies will be coupled with determination of the effects of the phospholipase C treatment on adrenergically stimulable adenylyl cyclase activity.